Luna Pro battery adapter $$$$

[BrianShaw]

I've never seen anyone other than Tom talk about a practical voltage regulator option. Most folks simply are not affected by the voltage drop-off or have ways of knowing when to replace their battery cells.

 

[Anon Ymous]

But the question remains:
Is there a practical alternative to "just" inserting a diode into the circuit as de Gruijter unfolds in his treatise?

A voltage regulator that nevertheless will work with those small differences in cell voltages?

There are many low dropout voltage regulators out there... Some with less than 0,1V voltage dropout, but that also depends on the load. The lower the load, the lower the voltage drop. For the few mA many light meters draw it will be very low.

 

[AgX]

So, one could produce a drain-time/voltage characteristic that lies even closer to the old mercury cell than those diode solutions.

What would be the disadvantages of such alternative way?

How big would such circuit be? I'm thinking of circuits employing SMDs.

 

[BrianShaw]

Wouldn't it be easier and cheaper to just replace batteries more often?

 

[Patrick Robert James]

I have a Luna-Pro and installed whatever diode or doohitchamajigger I was supposed to, then calibrated it with the calibration screws. Works great now and matches my Luna-Pro SBC perfectly even after a couple years. I have a bunch of meters though, so I rarely ever use it.

 

[pstake]

I sent a message to the gentleman who modified my Luna Pro, to ask about battery-replacement frequency. This is what he said:

I push the test button every couple months. I don't use it as many minutes in a shooting day as you will with large format. Just using it occasionally to touch up the camera readings, I get over a year out of the batteries. As long as the test is close to the test marking, your readings will be accurate. The silver oxide batteries burn off their surface charge after an hour or so of usage, and then they stay at their working voltage for 90% of their life. When they start to drop they drop sharply. So you don't have an extended period of decreasing voltage to mislead your readings. When I calibrate a luna pro with a new battery I set it at the top edge of the test mark. That way when it reaches its working voltage it'll show 1/2 to 2/3 of the test range. You'll notice when it starts down because the test reading will move toward the bottom of the test range. Battery temperature is a significant factor, so I keep it in my pocket in the winter.

When I was using a CdS luna pro as my reference meter for tuning other meters and camera meters I'd put several hours on it in a week. At that rate, I'd use up a set of batteries in about 3 to 4 months.

 

[BrianShaw]

That is very consistent with what Gossen has been saying for 25 years... At least. This really hasn't a been a major problem for most people..

 

[Tom1956]

I suppose many of the other ideas here other than mine would truthfully be good enough mostly. In my case, it was merely a project I wanted to tackle and win with a total victory. And I stayed with it till I was happy. I had a standard set in mind and was not going to be defeated. I had a luna pro, 2 sbc's and a sekonic. And my standard of perfection was 1/3 stop or better, full scale, and agreement with all 4 meters all the time, even till end of battery life. So, I adopted my would-be favorite, the weakling of the bunch, but the sweetest one of all--the original gray luna-pro, which had been rendered obsolete by nothing other than legislative action; the killing off of the PX625. I might add an ironic note, that the same kinds of thinkers who killed off the little bitty button battery for reasons of mercury; are the same ilk that killed off the light bulb to force people to switch to the curly light bulbs, crammed full of mercury. Non-sequitur.
But in the end, I've got the finest Luna-Pro of them all now. I don't always have such sweet successes with my hairbrain knucklehead project jags I get on.

 

[BrianShaw]

Don't let anyone squelch your dreams. Sometimes it is fun to solve problems even if nobody else understands.

 

[Anon Ymous]

So, one could produce a drain-time/voltage characteristic that lies even closer to the old mercury cell than those diode solutions.

What would be the disadvantages of such alternative way?

How big would such circuit be? I'm thinking of circuits employing SMDs.

I do not pretend to be an expert, but perhaps quiescent current could be a deal breaker. Given a large enough quiescent current and-or enough time, the regulator will drain the battery by itself, even if it doesn't provide any current to the load. Some regulators have a shutdown pin that decreases power consumption dramatically, but how can such a mechanism be used in a battery adapter? Ideally, the on-off switch of the device should power off the regulator, and solve the quiescent current problem. This of course means modifying the device, not making a battery adapter. Anyway, you could use a SOT23 regulator, which is small enough, but not too small to be (relatively easy) hand-soldered. That and probably a 1μF output capacitor are all you are going to need. A 0805 size capacitor would be fine and using 0,8mm thick pcb would make a quite thin tiny printed circuit board.

 

[tessar]

I have no technical expertise either, but I have practical experience. I used a Luna Six (= Luna Pro outside the US) for years with great results. Then the mercury cells were banned. A local camera store had the adapter for $40. This was years ago, so I think $60 is an OK price. I'm still using the meter, works as well as ever. If I were you, I'd spend the loot. I've never found a reason to replace the Luna Six. I like the possibility of using it under the light of the full moon, which gave the meter its name.

 

[Xmas]

But the question remains:
Is there a practical alternative to "just" inserting a diode into the circuit as de Gruijter unfolds in his treatise?

A voltage regulator that nevertheless will work with those small differences in cell voltages?

A pair of diodes is less than a dollar (USA).

Donno what Tom used in his regulator or what battery but a switching mode circuit can step voltage up or down.

But a regulator is more expensive eg the diode solution is 60USD commercially do you want to pay more?

The battery test switch and datum was necessary for mercury cells and is also sufficient for silver cells.

You need to read the link Tom posted again.

Fixing things that are not broken will get you fired.